Low-cost wireless mouse pays

While the ubiquitous computer mouse is hardly something that screams leading edge, the product's sheer necessity and volume have quietly driven some fantastic technology developments in this sometimes-overlooked peripheral. Longtime PC users will remember the days of hardwired mice and lint-clogged roller balls, but modern approaches to mouse design dictate cutting the cord and scrapping mechanical-motion translation.

Wireless links between pointing device and PC along with optical sensor technologies are rapidly becoming the norm for the lowly mouse, thanks to the virtues of high volume and the cost pressures characteristic of everything in the PC peripheral world.

When it comes to computer accessories such as the Microsoft optical wireless mouse analyzed here, an almost schizophrenic emphasis on integration emerges. While much of the key functionality gets implemented in an ASIC, the balance of the system is implemented with small-scale-but very inexpensive-components. Along with the flexibility to tune the more analog aspects of the design, the discrete-component approach builds on time-tested, low-cost technologies.

An optical sensor from ST is central to the design, representing an amazingly sophisticated piece of advanced mixed-signal CMOS integration. In essence, an LED illuminates the surface on which the mouse rests and a CMOS imager array combined with special processor circuitry "looks" at the surface to detect image shifts corresponding to mouse movement. The VV5353, which was developed specifically for Microsoft's line of IntelliMouse Optical mouse products, updates at a claimed 6,000 frames per second. All this in a part that likely sells for less than a couple of bucks.

Other components within the mouse include a 16-kbit E2PROM from Microchip to serve as the code store for the processing portion of the ST sensor. A Texas Instruments boost converter (TPS61012) translates the two parallel 1.5-volt alkaline batteries into appropriate supplies for both the logic parts and the high-intensity-illumination LED.

To create the wireless portion of the mouse, a frequency-shift-keying (FSK) FM radio operating in the 27-MHz public band is implemented in widely available commodity components.

Back at the USB-connected "basestation" dongle to the host PC, a receiver based on Samsung's S1T3361D01 FM radio subsystem receives and demodulates the FSK signals. Data streams are fed to a Freescale MC68HC908JB8 microcontroller, which also supports the USB interface. Together, these two parts and their associated discrete circuits receive the directional changes and clicks from the mouse, which are then translated to user inputs to the PC.

All in all, the system puts most of its development eggs in one basket-specifically the ST sensor. The remaining analog radio pieces are built around a discrete-intensive solution, which leverages commodity bits and pieces to keep costs down and design flexibility up.

With a total estimated cost of goods sold below $15, the attention to detail, integration-and selective disintegration-pay off. And the $50 retail/$30 street price still allows for some profit.

The only bad news is that the wireless link's reliability left a bit to be desired and its range was found to be at the low end of the claimed 2 to 6 feet.

David Carey, president of Portelligent (Austin, Texas; www.teardown.com), which produces teardown reports and related industry research on wireless, mobile and personal electronics